WIP: Use GtkBox with entry and button box as location entry

I assumed @jbrummer would look at it, since he asked on IRC, but he didn't read the reply, so opening a MR here.

Aside from reverting progress bar to the in-page one, another problem is that completion popup exactly matches the real entry width:

But the problem from #519 (closed) is fixed, and the crashes are seemingly fixed as well.